Temporomandibular joint/muscle disorders (TMJD) represent a family of conditions that present with pain in the temporomandibular joint (TMJ) and muscles of mastication. Chronic TMJD occurs mainly in young women and is strongly associated with elevated levels of psychological stress. Chronic TMJD patients display minimal signs of tissue injury and benefit little from conventional anti-inflammatory drug therapies. The symptoms of chronic TMJD suggest a central neural dysfunction or problem of pain amplification. By contrast, most animal models of TMJ nociception rely on overt inflammation and do not account for known risk factors (i.e., estrogen status and stress). In this application we will use an established model of psychological stress, the repeated forced swim test (FST), known to induce persistent hyperalgesia and will determine its effects on TMJ nociceptive processing in female rats under high and low estrogen conditions. We will test the hypothesis that changes in estrogen status and psychological stress act through the periaqueductal gray-rostral ventromedial medulla (PAG-RVM), the main supraspinal pain modulatory system, to influence TMJ nociception. Since serotonergic (5HT) mechanisms mediate a significant portion of PAG- induced effects on spinal systems, we will test if specific 5HT receptors are involved in modulation of TMJ nociceptive processing at the dorsal horn level. We will stimulate TMJ afferent fibers by injection of the non-inflammatory agent, ATP, and record single neuron activity at the trigeminal subnucleus caudalis/upper cervical dorsal horn region (Vc/C1-2), the principal site of termination for TMJ nociceptors. Masseter muscle electromyographic (EMG) activity will allow us to assess treatment effects on a peripheral behavioral correlate of TMJ nociception. Three Specific Aims are proposed. Aim 1 will determine if estrogen status alters PAG-induced modulation of TMJ-evoked unit activity, masseter muscle EMG and c-fos immunoreactive neurons at the Vc/C1-2 region in sham and FST-conditioned rats. Aim 2 will determine if local actions of 5HT receptors at the caudal brainstem level contribute to PAG-induced modulation of TMJ nociception. Aim 3 will determine if local actions of 5HT receptors alone at the caudal brainstem level modify the responses to TMJ stimulation independent of overt PAG stimulation. These studies will provide new information on the influence of estrogen status and psychological stress on the neurobiology of brainstem systems thought to be critical for TMJD pain.